(1) Field of the Invention
The present invention relates to a position acquiring apparatus for specifying the spatial position of a mobile station in a mobile communication system composed of a plurality of base stations fixed on the ground and a mobile station moving on the ground or water or in the air, or more particularly, to a position acquiring apparatus for a mobile communication system which is composed of a plurality of base stations for transmitting signals each including a predetermined synchronizing signal synchronized with an identical clock and an identifying signal identifying the originating base station, and a mobile station capable of simultaneously receiving signals transmitted from the base stations.
(2) Description of the Related Art
Recent development of mobile communication systems such as automobile radio telephone system and portable telephone system has come to demand further improvement in communication quality, as well as new added values.
Conventionally, in hand-off control wherein a mobile station switches communication from one base station to another, or in diversity reception control wherein an antenna of optimum reception condition is selected from among a plurality of antennas provided in a mobile station, the reception levels at which radio waves from base stations are received by the mobile station are used as control parameters. In the former control, if the reception level of the radio wave from a first base station with which the mobile station is communicating becomes smaller than a predetermined value, for example, the mobile station switches the communication to a second base station, the reception level of the radio wave from which is greater than the predetermined value, and the terminal of the opposite party in a fixed network also switches the connection from the first to the second base station. In the latter control, the mobile station receives the radio wave from an identical base station via a plurality of antennas provided therein, and continually selects an antenna of the highest reception level so that the selected antenna is used for the communication.
As shown in FIG. 10, however, a radio wave transmitted from a base station 101 can reach a mobile station 102 directly via a path 103 and also indirectly via pathes 105 and 107; namely, the radio wave may be diffracted by an obstacle 104 (path 105) or be reflected at an obstacle 106 (path 107) before reaching the mobile station 102. In such environment, as the mobile station 102 moves, the reception levels fluctuate drastically.
Thus, if hand-off control is performed based on such largely fluctuating reception levels, the terminal of the opposite party may select an erroneous base station, lowering the communication quality. To properly carry out diversity reception control based on greatly fluctuating reception levels, complicated control algorithm is required, which leads to prolonged processing time.